So far, the standard reference of the frequency of the FSS-IMC loop was the actuator efficiency of the NPRO PZT. That was measured by cavity scan of reference cavity using the sidebands as the ruler of frequency.
But it would not be acurate enough and difficult to estimate the error. So I estimated the AOM efficiency by measuring the peak frequency of the signal from AOM driver.

Measurement

Injected the DC signal to the AOM driver and change the offset voltage.
Measured the peak frequency by using network analyzer.(spectrum at 7.5V input is attached)

Result

A_aom = 5.064MHz/V, sigma_A_aom = 0.007MHz/V

the plot is attached.